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Eroica
2006-Nov-30, 08:57 AM
I am currently reading Meteors and Meteorites: Origins and Observations by Martin Beech (2006), a very interesting guide written with the serious amateur in mind. His description of what happens when a meteor-producing meteoroid passes through the Earth's atmosphere is at odds with what Phil writes in the book:


When a meteoroid begins to encounter molecules in the Earth's upper armosphere, at about 300km up, it becomes heated through direct surface collisions - essentially the kinetic energy of any colliding molecules is converted into heat energy at the surface of the meteoroid. The meteoroid will continue to heat up until a surface temperature of about 2,000C is reached. Once this temperature has been achieved, typically at an altitude of about 115km, material will begin to ablate from the meteoroid's surface, producing a trailing wake of electrons and atoms in an excited or ionized state. At this stage the meteoroid is beginning to lose mass and is also beginning to gradually slow down. Collisions between meteoric ions and atmospheric atoms result in the production of a plasma trail behind the meteoroid. The excited atoms in the plasma trail will eventually lose energy through the emission of photons, and the electrons will also eventually recombine with the ions to produce more photons. It is the emission of these photons that produces a meteor's light. Spectroscopic observations of meteor trails indicate that it is mainly atoms and ions from the meteoroid that produce the observed light - Martin Beech
When the meteoroid enters the upper reaches of the Earth's atmosphere, it compresses the air in front of it. When a gas is compressed it heats up, and the high speed - perhaps as high as 100 kilometers per second - of the meteoroid violently shocks the air in its path. The air is compressed so much that it gets really hot, hot enough to melt the meteoroid. The front side of the meteoroid - the side facing this blast of heated air - begins to melt. It releases different chemicals, and it's been found that some of these emit very bright light when heated. The meteoroid glows as its surface melts, and we see it on the ground as a luminous object flashing across the sky. The meteoroid is now glowing as a meteor.

... In reality, there is actually very little friction between the meteoroid and the air. - Philip Plait
It would be nice to know a few more details about those meteoroid chemicals that glow when heated. Phil's description is a little vague at this point.

Fazor
2006-Nov-30, 09:00 PM
I had always been taught the first explination. But when I used that in a reply to someone's question, was told that I was wrong and that the combustion was actually caused by the second explination. Since these two ideas are at odds, I think maybe i'll just say it's some huge government/NASA coverup and that it's actually being shot by an alien defense lazer beam they set up to save us from unhealthily large meteor impacts. (not all aliens want to kill us, duh). :)

okay but seriously, which statement above is correct? or is it like so many other things, and it's a little of each?

Jeff Root
2006-Nov-30, 09:11 PM
The heating definitely starts out as direct collisions between the
meteoroid and the air. At some point, gas-gas collisions are likely
to predominate, but I'd like to see some kind of support for that
idea.

-- Jeff, in Minneapolis

ozark1
2006-Dec-01, 09:18 AM
You could consider how big these meteors are. If the light emission profile had anything to do with the vapourising chemicals, most meteors would be very dim indeed.

Most meteors are the same colour - green/yellow - which is consistent with the glow of an oxygen/nitrogen plasma (first explanation). Very big meteors/fireballs do get a red head - which is the second explanation.

The Bad Astronomer
2006-Dec-03, 06:27 AM
For my book I talked with John Lewis, who wrote "Rain of Iron and Ice" and is an expert on such things. As I recall (the conversation was like 7 years ago now) direct collisions are what start things off, but as soon as a shock wave builds up you get a standoff shock a few meteroid radii in front (so if it's pea-sized, it's a few cm in front). The heat ionizes the air, and there was one particular chemical (I don't recall which one) that recombines and glows very brightly.

I think that meteor spectra show the presence of ionized air and ablated meteoroid material.

ozark1
2006-Dec-03, 08:56 AM
Could try here http://www.asu.cas.cz/~borovic/leonid.htm

Pretty well shows the green oxygen line, various atmospheric lines and some Fe from the meteor. I'm not sure where the Na/Mg comes from but it is significant to the light intensity

Eroica
2006-Dec-03, 09:14 AM
Thanks, Phil. That clarification certainly brings the two explanations closer together. From reading your book, I understood that recombination of ionized atoms was not an issue - the word "ionization" is not mentioned - but clearly it is a crucial factor.

The only significant discrepancy between the two explanations, so far as I can see, is that Beech fails to acknowledge the compression of air in front of the meteoroid (and the creation of an associated standoff shock) as the principal source of heat. Both explanations attribute the light to recombination fo ionized atoms, and both identify meteoroid material (as opposed to atmospheric atoms) as the principal source of that light.

Eroica
2006-Dec-03, 09:20 AM
Could try here http://www.asu.cas.cz/~borovic/leonid.htm (http://www.asu.cas.cz/%7Eborovic/leonid.htm)

Pretty well shows the green oxygen line, various atmospheric lines and some Fe from the meteor. I'm not sure where the Na/Mg comes from but it is significant to the light intensity

From the abstract (http://www.asu.cas.cz/%7Eborovic/manuscri.htm):


Relative bulk abundances of Mg, Fe, Ca, and Na in Leonids are nearly CI-chondritic within the uncertainty of the method (factor of three). Smaller meteoroids tend to be poorer in sodium, which is true also for Perseids. Most meteoric vapor emissions could be reasonably well explained with the temperature of 4500 K. High temperature meteoric emissions (Ca+, Mg+) are present only in bright meteors. Leonid spectra are very rich in atmospheric emissions of O, N, and N2 even at high altitudes and in faint meteors. These emissions are therefore not connected with meteor shock wave.

Eroica
2006-Dec-12, 03:58 PM
An update:


That meteors produce emission line spectra indicates that the light we detect from them must be derived from a hot gas, and not from an incandescent solid body (although the atoms and molecules that make up the meteor "gas" do of course come from the meteoroid itself) ... It is clear that the emission lines are derived from a hot gas, at a temperature of about 4,000C, that surrounds the ablating meteoroid. Emission lines from many diverse atoms and molecules have been observed in meteor spectra. In most spectra the strongest lines are due to the so-called H and K lines of ionized calcium, and lines due to sodium and magnesium. Spectra obtained from very-high-speed meteoroids (such as those that produce Leonid meteors) also show emission lines due to atmospheric oxygen atoms [O] and nitrogen molecules [N2].

hhEb09'1
2007-Feb-15, 02:12 AM
An update:
(although the atoms and molecules that make up the meteor "gas" do of course come from the meteoroid itself)So, it would appear, that it is the meteor itself glowing, not the atmospheric gases that it compresses.

PS: maybe we can get an opinion from the resident heliochromologist?